In the past, a screen printing method was prevalently adopted for forming resist patterns on printed circuit boards. The screen printing technique involves squeezing a composition through the open meshes of a stretched piece of material such as wire onto a printable substrate. The screen is covered or blocked out in part by a masking material in order to form the desired pattern on the printable substrate. The masking material may simply be a stencil or a dried lacquer, shellac or glue. Once the screen has been covered or blocked out in part by a masking material, it is held taut on a frame and positioned over the desired substrate. A composition is then poured onto the screen and squeezed through the open areas, as with a squeegee. It is important, when following a desired pattern, that the composition does not flow or bleed outside of the preselected areas defined by the open areas of the screen, but should follow accurately the image formed on the screen and reproduce it.
The screen printing method by nature has low resolution. When screen printing is carried out with a screen printing ink of relatively high viscosity, such adverse phenomena as breaks, screen marks, and pinholes occur in the produced patterns. When screen printing is carried out with a screen printing ink of relatively low viscosity, such adverse phenomena as bleeds, smudges and sags ensue. Because of these defects, screen printing can no longer keep abreast with the recent trend of printed circuit boards toward increasing density.
Printed circuit boards have also been prepared by a process which comprises applying copper plating onto a copper foil plated, laminated plate, laminating thereonto a photosensitive film, exposing the photosensitive film to light through a photographic negative, and removing the unexposed portion, followed by etching away any unnecessary copper foil part not under a circuit pattern, and by removing the photosensitive film to form a printed circuit on the insulated laminated plate. The photosensitive film used in this process raises the problem that the circuit pattern formed by exposing to light and developing is not sharp because the film is so thick, normally in the neighborhood of 50 .mu.m, that it is difficult to uniformly laminate the photosensitive film on the surface of the copper foil. The photosensitive film is mostly removed uselessly in spite of being expensive.
Further, variable information such as production date, lot number, batch number, serial number, and the like, are presently placed on individual circuit boards by hand, using so-called legend inks.
Ink jet printing and ink compositions which permit ink to be jetted from an ink jet printer offer a means for preparing circuit boards of increasing density and printing variable information or a circuit board. Ink jet printing is a non-impact technique for projecting droplets of ink onto a substrate. There are two major categories of ink jet printing, "Drop-On-Demand" ink jet and "Continuous" ink jet. Using Drop-On-Demand ink jet technology, the ink is normally stored in a reservoir and delivered to a nozzle in the print head of the printer. A means exists to force a single drop of ink out of the nozzle whenever it is needed to print a single spot on the printed medium (for example, paper). For Continuous ink jet, a conducting ink is supplied under pressure to an ink nozzle and forced out through a small orifice, typically 35 to 80 .mu.m in diameter. Prior to passing out of the nozzle, the pressurized ink stream proceeds through a ceramic crystal which is subjected to an electric current. This current causes a piezoelectric vibration equal to the frequency of the AC electric current. This vibration, in turn, generates the ink droplets from the unbroken ink stream. The ink stream breaks up into a continuous series of drops which are equally spaced and of equal size. Surrounding the jet, at the point where the drops separate from the liquid stream in a charge electrode, a voltage is applied between the charge electrode and the drop stream. When the drops break off from the stream each drop carries a charge proportional to the applied voltage at the instant at which it breaks off. By varying the charge electrode voltages at the same rate as drops are produced it is possible to charge every drop to a predetermined level. The drop stream continues its flight and passes between two deflector plates which are maintained at a constant potential, typically +/- 2.5 kV. In the presence of this field, a drop is deflected towards one of the plates by an amount proportional to the charge carried. Drops which are uncharged are undeflected and collected into a gutter to be recycled to the ink nozzle. Those drops which are charged, and hence deflected, impinge on a substrate traveling at a high speed at right angles to the direction of drop deflection. By varying the charge on individual drops, the desired pattern can be printed.
The ink jet process is adaptable to computer control for high speed printing of continuously variable data. Ink jet printing methods can be divided into three general categories: high pressure, low pressure and vacuum techniques. All have been described and employed in conventional ink jet printing and can be employed in the present invention.
Reviews of various aspects of conventional ink jet printing can be found in these publications: Kuhn et al., Scientific American, April, 1979, 162-178 and Keeling, Phys. Technol., 12(5), 196-303 (1982). Various ink jet apparatuses are described in U.S. Pat. No. 3,060,429, U.S. Pat. No. 3,298,030, U.S. Pat. No. 3,373,437, U.S. Pat. No. 3,416,153 and U.S. Patent No. 3,673,601.
German Patent Specification No. 3,047,884 discloses the preparation of printed circuit boards employing an ink jet printer. Also disclosed is the spraying of organometallic solutions such as organocopper compounds directly onto an unmetallized circuit board. By means of a subsequent laser beam, the conductor pathways can be cured completely.
Vest et al., Int'I J. Hybrid Microelectronics, 6, 261-267 (1983), discloses computer controlled ink jet printing of hybrid microelectronics circuits. An ink jet printer is used with a conductor ink based on metallo-organic compounds in solution. The use of silver neodecanoate as a silver conductor is disclosed, with or without added platinum in the form of platinum amine octoate, to produce a solderable connector. Silver conductor line patterns on glass and alumina substrates were produced, the silver inks decomposed to silver when heated to 250.degree. C.
U.S. Pat. No. 4,668,533 discloses the preparation of printed circuit boards using an ink jet printer by depositing a water-based ink which contains a metal on a substrate in a predetermined pattern, followed by the depositing of a second metal to the same substrate congruent to the first metal.
Japanese KoKai No. 66089/1981 discloses a printed circuit board obtained by ink jet printing onto a copper clad plastic substrate, an acid-proof ink containing a material such as silicon varnish which solidifies on drying, and etching the undesired portion of the copper foil. The silicon varnish has several disadvantages. The silicon varnish is sensitive to hydrolysis by water in a humid environment which would result in an ink having different properties than it did prior to hydrolysis. Further, as a solvent based system, the printed message dries quickly preventing any changes from being made. In contrast, the ink composition of the present invention is not set until it is exposed to UV light. Therefore, changes can easily be made by wiping off the uncured message with a dry towel and printing again.
A need therefore exists for a UV curable, etch-resistant ink composition capable of being jetted from an ink jet printer onto a metal or ceramic substrate that does not suffer from the drawbacks of the silicon varnish ink. A further need exists for a method for applying variable information to a printed circuit board. The foregoing and other needs are satisfied by the present invention and will be apparent from the description of the invention provided herein.